The development of portable electronic equipment, in general, and mobile phones, in particular, has revealed a significant interest for the use of Switched Mode Power Supply (SMPS) systems.
Indeed, such SMPS systems, contrary to linear regulators, achieve a high efficiency—on the order of 90 or 95%—which may be well beyond what is allowed by linear voltage regulator. Therefore, electronic products manufacturers tend to use SMPS systems in a wide variety of portable devices, including mobile phones, MP3 players, Portable Device Assistant, etc.
Consider, for instance, the particular case of mobile phones. These devices tend to incorporate many multimedia functions, including specific audio capabilities. A mobile phone may now serve as a MP3 player and thus incorporate an audio amplifier which, for the purpose of saving the battery life, may be class AB or even class G, using a voltage supply which is adjusted in accordance to the level of the audio signal to amplify.
FIG. 1 illustrates a schematic view of such an audio device 10 intended to drive a headset 15, comprising two drivers, using a stereo AB class power amplifier 13. Power amplifier 13 receives analog signals which are converted by DAC 14 into analog form from the digital audio files stored within memory (not illustrated). The powering of the class AB amplifier is achieved by the combination of a dedicated SMPS unit 11 providing the positive voltage, for example a voltage between 2.2V and 1.2V, in accordance with the amplitude of the analog audio signal to be amplified. In addition, a negative charge pump 12 coupled to an appropriated fly capacitor 16 and tank capacitor 17 provides the negative voltage.
This structure achieves, thanks to the combination of the SMPS 11 and the negative charge pump 12, a very high efficiency on the whole range of amplitude on the audio signal. Indeed, the envelope detection of the audio signal gives the code for the control bits (VSEL<3:0>) to adjust the both SMPS and Negative Charge pump output voltages according the audio signal.
This system achieves the possibility of creating a true H CLASS supply modulation with several levels with a real gain of efficiency on various levels. A limitation of such a system results from its use of a dedicated SMPS unit, having a dedicated and expensive coil in addition to the additional area required on the semiconductor substrate.
A second system is described in U.S. Pat. Pub. 2013/0214,852, assigned to the assignee of the present disclosure, entitled “Dual output charge pump generating two voltage values with two distinctive levels and control method thereof”, which is represented in FIG. 2, and hereby incorporated by reference in its entirety. This second system allows removal of the costly dedicated SMPS device of FIG. 1 by using a generic SMPS device already present in the chip or in the platform (for instance a specific SMPS powering Input/output interface with a fixed voltage of 1.8 Volts). Thanks to a dual output charge pump (positive and negative), using a second fly capacitor, the possibility of generating two levels of positive voltages (VDD and VDD/2) and corresponding two levels of negative voltages (−VDD and −VDD/2) is achieved.
FIG. 2 illustrates the schematical view of an audio device which would implement the second system known from U.S. Ser. No. 13/816,349. For the sake of conciseness, the elements which are common with those of FIG. 1 bear the same numerals references (headset 13, amplifier 13, fly and tank capacitor 16 and 17). The SMPS circuit 21 provides a fixed output voltage (1.8V for instance), which is used for powering a dual positive and negative charge pump 22 associated with two fly capacitors, respectively 16 and 26, and two tank capacitors, respectively 17 and 27.
The second system depicted in FIG. 2 achieves the possibility of providing two voltage levels (±1.6V/±0.75V) and thus a true G-Class system, without using dedicated SMPS and thus reducing the manufacturing costs.
However, it is still desirable to provide better efficiency and performance. Therefore, further developments in this area are desired.